1. Field of the Invention
The present invention relates to a conductive paste used to adhere a heating element such as a chip constituting an electronic part to a radiator such as a lead frame, and to an electronic part such as a semiconductor device to which a chip is adhered using this conductive paste.
2. Description of the Prior Art
Conventionally a hard solder is mainly used to adhere to and mount on a metal plate such as a lead frame, a chip constituting an electronic part such as a semiconductor device. A hard solder used as a mounting material (a die bonding material) generally comprises a eutectic alloy such as Au-Su (gold-tin) or Au-Sb (gold-antimony). Such a hard solder can efficiently transmit to the metal plate, heat generated in the chip, thereby enabling electronic parts with an excellent radiating capability to be produced.
Due to its high melting point, this hard solder must be heated to a high temperature in adhering the chip. Thus, if the chip comprises a semiconductor such as GaAs having a low heat resistance, it may have its properties degraded or may even be destroyed. In addition, due to the difference in thermal expansion coefficient between the hard solder and the chip, a stress may occur after mounting, resulting in warps or cracks. Besides, this hard solder is likely to cause voids to remain between the chip and the metal plate during adhesion. Moreover, to achieve appropriate mounting, a hard solder containing gold must be used but such a solder is very expensive. Besides, to achieve mounting without residual voids, skills are required to place and press the chip appropriately, so it is difficult to automate mounting with a hard solder.
Thus, the hard solder has many problems, but it is still used for electronic parts likely to become very hot, due to its high thermal conductivity.
Attempts are being made to use as a mounting material a silver paste consisting of silver (Ag) powder, a thermosetting resin, and a solvent, instead of the hard solder. This silver paste can transmit heat appropriately after adhesion and hardening due to the high thermal conductivity of silver, but is unlikely to thermally degrade the chip during mounting due to the relatively low hardening temperature of the mixed resin. In addition, the mixture of the resin serves to restrain stress, and costs are relatively low due to the prices of the mixed silver and resin lower than that of gold. Moreover, the amount of viscosity control agent (solvent) added can be adjusted to control the viscosity or thixotropy and handling the silver paste is easy, so a predetermined amount of paste can be selectively added to an adhesion surface by means of printing coating, injection, or dripping in order to allow the chip to be mounted appropriately, thereby enabling automation to be realized relatively easily.
Silver powder contained in such a silver paste has scale-shaped particles. Silver powder of scale-shaped particles contributes to improving the spreadability of the paste, thereby enabling it to be applied to the adhesion surface more easily. Since the silver powder serves to reduce the settling velocity of silver within the paste, the number of times that the paste is stirred during preparation or prior to use, thereby improving the operability during preparation or mounting and providing homogeneous paste to enable appropriate mounting. Furthermore, it serves to improve the shear strength after mounting to prevent the paste from being peeled off against a horizontal external force.
Such a silver paste, however, has a lower thermal conductivity (higher heat resistance) than the hard solder after adhesion and hardening, so electronic parts obtained using it have a low radiating capability. This is due to the small number of contacts among silver particles resulting in a small contact area or the orientation of scale-shaped silver particles leading to the variation of quality.
In general, to increase the thermal conductivity (reduce the heat resistance) of a silver paste after adhesion and hardening, the amount of silver powder in the paste may be increased. Too much silver powder, however, reduces the relative mixing amount in the paste of resin having an adhesive property to degrade the adhesive property of the silver paste. In addition, an increase in the amount of silver powder mixed increases the viscosity of the silver paste, resulting in the need to mix a large amount of solvent. As a result, after adhesion and hardening, the solvent may remain in a silver paste layer (an adhesive layer) or voids may occur therein, thereby degrading the properties such as thermal and electric conductivities and the reliability of the electronic part.
Japanese Patent Publications disclose the following two techniques as silver paste improvement techniques.
Japanese Patent Application Laid-Open No. 62-79635 discloses a silver paste containing metal balls larger than silver particles. Specifically, Cu balls of 50 to 200 xcexcm particle size are mixed in the silver paste containing silver particles of 1 to 3 xcexcm particle size. This application states that 10 to 50 vol. % of metal balls (Cu balls) can be effectively mixed in a silver paste containing 80 to 90 wt. % of silver particles after a solvent has been volatilized (cured). It indicates that the mixture of metal balls eliminates the variation of thermal conductivity while reducing the heat resistance from 90 to 85xc2x0 C./W.
This silver paste, however, contains 10 to 50 vol. % of metal balls (Cu balls) of large particle size 50 to 200 xcexcm, so when this paste is used for adhesion, the silver paste layer (the adhesive layer) becomes thick after adhesion and hardening. It is clear that with this size of metal balls, the thickness of the adhesive layer will be several hundred xcexcm or more. Since the thickness of lead frames or chips is being continuously reduced down to about 200 xcexcm, such an increase in the thickness of the adhesive layer is against the recent demand for the reduced size of packages and the reduced thickness of films. In addition, this application states that the metal balls prevent the thermal conductivity from varying and improve it, but does not describe the electric conductivity. The electric conductivity can be assumed to decrease because an increase in particle size generally reduces the number of contacts among the particles and thus the contact area.
Japanese Patent Application Laid-Open No. 7-201896 discloses a silver paste characterized in that it is composed of silver powder, a thermosetting resin, and a viscosity control agent and in that the silver powder consists of scale-shaped silver powder to which spherical silver powder of average particle size 5 to 10 xcexcm is added. This application states that the average particle size of the scale-shaped silver powder is desirably between 4 and 10 xcexcm and that the amount of spherical silver powder added is 5 to 50 wt. % of the total amount of silver powder.
The problems that this application for the silver paste attempts to solve are shown below. Due to a large amount of scale-like Ag powder contained, the die bonding material currently used (silver paste) has a reduced adhesive strength, is likely to be destroyed at the adhesion interface, and cannot be uniformly manufactured. Moisture attached to voids present in a die bonding layer or a control agent (a solvent) remaining in the layer may cause cohesive failure during molding process or package mounting, leading to package cracks. Japanese Patent Application Laid-Open No b. 7-201896 attempts to solve these problems.
The affects of this silver paste are described as follows. This invention (the above silver paste) replaces some of the scale-shaped Ag powder by larger spherical Ag powder as a method for solving the above problems. That is, the die bonding material currently used contains as large an amount of scale-shaped Ag powder as possible to improve its radiating capability, and due to its large area per unit weight, the scale-shaped Ag powder noticeably contributes to increasing the viscosity. In addition, an increased amount of viscosity control agent (a solvent) is added to regulate the viscosity. In addition, due to the reduced junction area of the thermosetting resin at the adhesion interface, the adhesive strength decreases. Thus, when some of the scale-shaped Ag powder is replaced by larger spherical Ag powder, the viscosity is precluded from increasing and the junction area of the thermosetting resin at the adhesion interface is increased to improve the adhesive strength.
This silver paste, however, is intended to improve the viscosity and adhesion strength as described above, and this application does not describe or suggest the improvement of thermal conductivity. In addition, since an increase in particle size generally reduces the number of contacts among the particles and thus the contact area, the above silver paste containing the larger spherical Ag particles is not expected to improve the thermal conductivity after adhesion and hardening or the radiating capability of electronic parts produced using this paste. In addition, this application neither describes the electric conductivity of the silver paste, and since an increase in particle size generally reduces the number of contacts among the particles and thus the contact area, the above silver paste containing the larger spherical Ag powder is not expected to improve the electric conductivity, as in the thermal conductivity. Furthermore, due to the use of the larger spherical Ag powder in the silver paste, the reduction of the thickness of films is limited.
It is thus an object of this invention to provide a conductive paste that provides practically sufficient junction strength and paste viscosity, that provides high thermal and electric conductivities after adhesion and hardening, and that enables an adhesive layer to be formed thinner. It is another object of this invention to provide an electronic part produced using this conductive paste and having a high radiating capability and excellent electric characteristics.
By making various efforts to achieve these objects, the inventors have completed the present invention.
The present invention relates to a conductive paste of a high thermal conductivity characterized in that a conductive paste containing at least conductive particles, a hardenable resin, and a viscosity control agent has been mixed with fine spherical conductive particles smaller than the first conductive particles.
The present invention also relates to an electronic part composed of a heating element and a radiator adhered and fixed together via an adhesive layer characterized in that the adhesive layer consists of at least a mixture of conductive particles and a resin and in that the conductive particles are mixed in such a way that conductive particles of a smaller particle size are filled between conductive particles of a larger particle size.